To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) "X extends Y" is correct if and only if X is a class and Y is an interface - This option is incorrect. In Java, the keyword "extends" is used to indicate that a class is inheriting from another class or implementing an interface. Therefore, "X extends Y" is correct if X is a class and Y is either a class or an interface.
Option B) "X extends Y" is correct if and only if X is an interface and Y is a class - This option is incorrect. In Java, interfaces cannot directly inherit from classes. Instead, they can extend other interfaces. Therefore, "X extends Y" is incorrect if X is an interface and Y is a class.
Option C) "X extends Y" is correct if X and Y are either both classes or both interfaces - This option is correct. In Java, both classes and interfaces can extend other classes or interfaces. Therefore, "X extends Y" is correct if X and Y are either both classes or both interfaces.
Option D) "X extends Y" is correct for all combinations of X and Y being classes and/or interfaces - This option is incorrect. As mentioned before, interfaces cannot directly extend classes in Java. Therefore, "X extends Y" is not correct if X is an interface and Y is a class.
The correct answer is C. "X extends Y" is correct if X and Y are either both classes or both interfaces.